session expire after 7days in php magento

$expire=time()+60*60*24*7;
        if (isset($_COOKIE["slideLastCookies"])) {
            $cookies_exist = true;
             for($counter=2;$counter<=6;$counter++)  {
            if(Mage::getModel('cms/block')->load('news_block_'.$counter)->getIsActive()) { ?>
                <div class="postContent slide_<?php echo $counter; ?>">
                    <?php echo $this->getLayout()->createBlock('cms/block')->setBlockId('news_block_'.$counter)->toHtml(); ?>
                </div>
            <?php
            }
        }
        } else {
            $cookies_exist = false;
            setcookie("slideLastCookies", time(), $expire);
            for($counter=1;$counter<=6;$counter++)  {
            if(Mage::getModel('cms/block')->load('news_block_'.$counter)->getIsActive()) { ?>
                <div class="postContent slide_<?php echo $counter; ?>">
                    <?php echo $this->getLayout()->createBlock('cms/block')->setBlockId('news_block_'.$counter)->toHtml(); ?>
                </div>
            <?php
            }
        }
        }